stack element

例句：

1. 1. The boxes are stacked neatly on the shelves.
   盒子整齐地堆放在架子上。
2. 2. The books were stacked on the desk.
   书被堆放在桌子上。
3. 3. The boxes are being stacked in the warehouse.
   箱子正在仓库里堆放。